1. Field of the Invention
The invention relates to a circuit arrangement for a tuner, particularly a television tuner, for a change over switching between two frequency bands with the aid of switching diodes, said frequency bands being filtered out by means of band-pass filters having at their inputs and outputs tuning capacitance diodes, the switching diodes of said two bandpass filters being alternately forward-biased.
2. Brief Description of the Prior Art
In circuit arrangements of this type it is known to provide a tunable circuit with a capacitance diode at each input and output of each band-pass filter. The secondary sides of the bandpass filters are then connected to inputs of a mixer, generally an integrated mixercircuit. A switching possibility for the output signals of these bandpass filters must be provided at these inputs, because only the the signal of one of the bandpass filters should reach the mixer. These circuit arrangements generally comprise switching diodes which are rendered conducting by means of a switching signal so that the output signal of the bandpass filter is short-circuited to ground.
A tuned input stage for three frequency bands is known, for example, from U.S. Pat. No. 4,408,348 which has three tunable filters for each of the three frequency bands. At the inputs and outputs of each of the three filters, tunable circuits, each having a capacitance diode are provided.
Furthermore a tunable resonant circuit for at least two frequency ranges is known from DE-AS No. 25 03 785 in which, for example, a common tunable resonant circuit is provided for the frequency ranges UHF, VHF I and VHF III. However, in this resonant circuit the highest band, in this example the UHF band, is continuously operative. This results in an interference signal being produced in the UHF range, for example, in the case of reception in the UHF III band owing to the occurrence of harmonics, which interference signal, likewise as the wanted signal reaches the mixer.